1. Field of the Invention
The present invention relates to a method for treating ammoniacal nitrogen in wastewater, and particularly relates to a method for nitrifying ammoniacal nitrogen contained in wastewater via biological oxidation and producing nitrite nitrogen and nitrate nitrogen at a desired rate.
2. Description of the Related Art
Wastewater containing nitrogenous components is a key factor in promoting eutrophication in closed water areas to cause water pollution. Hence, a nitrifying treatment is carried out by biologically decomposing and removing nitrogenous components in wastewater via using bacteria in some sewage treating facilities and wastewater treating facilities.
Conventionally, a nitrifying-denitrifying treatment performed by combination of nitrifying and denitrifying treatments has been widely used as a method for biologically treating wastewater containing nitrogenous components. In the nitrifying-denitrifying treatment, ammoniacal nitrogen contained in to-be-treated water is oxidized up to nitrate nitrogen by a nitrifying bacterial population, subsequently the resultant nitrate nitrogen is reduced to nitrogen gas by denitrifying bacteria, whereby nitrogenous components are eventually removed.
On the other hand, the anaerobic ammonium oxidation (ANAMMOX) method has been recently developed to be practically usable. The anaerobic ammonium oxidation reaction is a reaction in which ammonia and nitrite are co-denitrified under the anaerobic conditions, and represented by the following chemical formula (1).1.00NH4++1.32NO2−+0.066HCO3−+0.13H+→1.02N2+0.26NO3−+0.066CH2O0.5N0.15+2.03H2O  Formula (1)
The anaerobic ammonium oxidation reaction is performed by autotrophy anaerobic ammonium oxidizing bacteria using ammonia as a hydrogen donor. This reaction has advantages that feeding of a carbon source such as methanol is not needed thereto, resulting in suppression of the operation cost in a low level. Further, it is not needed to oxidize nitrite nitrogen to nitrate nitrogen, which reduces the power cost required for aeration. Moreover, the anaerobic ammonium oxidizing bacteria have a high denitrifying rate, while having a low growth yield. This feature contributes to maintenance of the process efficiency, and reduction of the equipment scale, leading to an advantage for reducing the excess sludges.
Wastewater containing nitrogenous components generally includes ammoniacal nitrogen in many cases. On the other hand, in the anaerobic ammonium oxidation reaction, ammonium reacts with nitrite at the rate of approximately 1:1.3 as represented by Formula (1). Therefore, in the anaerobic ammonium oxidation method, nitrite type nitrification is performed so that a part of ammoniacal nitrogen is oxidized to nitrite nitrogen.
A method for treating ammoniacal nitrogen by the anaerobic ammonium oxidation method is categorized in two techniques. One is a single-tank technique which uses a single-tank configured to carry out nitrite nitrification and anaerobic ammonium oxidation. The other is a two-tank technique which uses an ammonium oxidizing tank configured to carry out the nitrite nitrification and an anaerobic ammonium oxidation reaction tank configured to carry out the anaerobic ammonium oxidation.
The single-tank technique includes a CANON method carried out under the aeration conditions in which the oxygen concentration is restricted at a low level, an OLAND method carried out under the conditions in which the oxygen concentration is restricted at a low level, and an SNAP method carried out by growing the anaerobic ammonium oxidizing bacteria inside a carrier which sticks and fixes a nitrifying bacterial population therein.
Further, the two-tank technique includes a one-pass method for introducing all amounts of to-be-treated water into the ammonium oxidizing tank so as to partially nitrite-oxidize a part of ammoniacal nitrogen to nitrite nitrogen, and a by-pass method for introducing a part of to-be-treated water into an ammonium oxidizing tank so as to nitrate-oxidize all of the ammoniacal nitrogen to nitrate nitrogen, while diverting the remained part of the to-be-treated water to be joined to the treated water.
Generally, a bacterial sludge containing a nitrifying bacteria population is used for the nitrite nitrification which oxidizes ammoniacal nitrogen to nitrate nitrogen. The nitrifying bacteria are typically a mixture of ammonium oxidizing bacteria (AOB) which oxidize ammoniacal nitrogen to nitrite nitrogen and nitrate oxidizing bacterial (NOB) which oxidize nitrite nitrogen to nitrate nitrogen.
Hence, both the methods for treating ammoniacal nitrogen are demanded to control progress of the nitrite nitrification, so that the oxidation of ammoniacal nitrogen comes to stop to nitrite nitrogen, and the operation is carried out by keeping the production amount and rate of the nitrite nitrogen.
Generally, it is well known that operation of the nitrite nitrification using a nitrifying bacteria population is not easily continued while stably keeping the production amount of nitrate nitrogen. In a typical water quality, nitrite oxidizing bacteria tend to grow and proliferate. Thus, in many cases, nitrite nitrogen produced by the ammonium oxidizing bacteria is rapidly oxidized to nitrate nitrogen, and therefore, it is difficult to control the oxidation of the ammoniacal nitrogen within the partial oxidation stopping to the nitrite nitrogen. Hence, various methods have been investigated to control the production amount of nitrate nitrogen.
For example, Japanese Patent Publication No. 5292659 discloses a nitrifying method performed by adjusting an aeration flow rate of a nitrifying tank thereby to control the nitrification to be nitrite-type nitrification.
Further, Japanese Unexamined Patent Application Publication No. 2005-131452 discloses a nitrifying method for treating wastewater containing ammoniacal nitrogen. In this method, a nitrifying tank is divided to two tanks of first and second nitrifying tanks, and the nitrifying conditions of the first nitrifying tank are controlled so that to-be-treated water in the first nitrifying tank has a free ammonium concentration higher than the inhibitory concentration to inhibit the bioactivity of nitrite oxidizing bacteria.
Moreover, Japanese Patent Publication No. 3788601 discloses a method for manufacturing a nitrite-type nitrifying carrier which preferentially accumulates ammonium oxidizing bacteria. For example, it is described that a method is performed by inclusively-immobilizing a sludge with a monomer or a prepolymer used for immobilizing ammonium oxidizing bacteria, and subsequently heating the resultant material at the temperature ranging from 60° C. to 80° C. for the polymerization. Further, it is described that another method is performed by heating a monomer or a prepolymer used for immobilizing bacteria in the presence of the sludge for at least one hour for the polymerization.
A method for controlling a production amount of nitrite nitrogen within a target one includes a technique performed by measuring a total nitrogen concentration or an ammoniacal nitrogen concentration as disclosed in ISAKA, K. et al. “Novel autotrophic nitrogen removal system using gel entrapment technology”, Bioresource Technology, 2011, 102, pp. 7720-7726, and adjusting a dissolved oxygen concentration via referring to a nitrite oxidizing rate (ηNit) as a standard value calculated by the total nitrogen concentration or the ammoniacal nitrogen concentration thus measured.
For example, ISAKA et al., discloses that a total nitrogen concentration or an ammoniacal nitrogen concentration is measured in a sensing tank provided at a downstream side of a nitrite-oxidizing tank.
As disclosed in Japanese Patent Publication No. 5292659 and Japanese Unexamined Patent Application Publication No. 2005-131452, there is a technique for controlling a production amount of nitrite nitrogen via adjusting a dissolved oxygen concentration and an ammoniacal nitrogen concentration in a nitrifying tank. However, a method for controlling simply an aeration flow rate alone as disclosed in Japanese Patent Publication No. 5292659 and a method for controlling simply a free ammonium concentration alone as disclosed in Japanese Unexamined Patent Application Publication No. 2005-131452 have a disadvantage so that the responsiveness and accuracy of the control are poor.
For example, when a dissolved oxygen concentration becomes high or a free ammonium concentration becomes low even in a temporary or local way, this change in concentration allows nitrite nitrogen once produced to be oxidized in turn. Accordingly, nitrate nitrogen is extremely accumulated in a typical retention period, letting the production amount of nitrite nitrogen hardly kept within the target one.
Further, biological oxidation of ammoniacal nitrogen contained in wastewater is demanded to be performed so that production amounts of not only nitrite nitrogen but also nitrate nitrogen are controlled within the target amounts. For example, a combining use of heterotrophic bacteria which reduce nitrate nitrogen to nitrite nitrogen and anaerobic ammonium oxidizing bacteria is demanded to produce a stable amount of nitrate nitrogen. However, it is hard to actively control a production amount of nitrate nitrogen by the methods disclosed in the above three patent documents. Further, the method for controlling a dissolved oxygen concentration as disclosed in ISAKA et al. requires a high level of technology and numerous sensors, failing to be a method suitable for the practical use.